Centrifugal compressors are used in a wide variety of industrial applications. For instance, centrifugal compressors are used in the oil and gas industry, for boosting the pressure of hydrocarbon gases. The compression work required for compressing gas through the rotating impellers and the diffusers of a centrifugal compressor generates an axial thrust on the compressor shaft. Balancing drums are often used for reducing the total axial thrust on the shaft bearings.
Some known compressors have a so-called back-to-back configuration, which reduces the axial thrust on the compressor shaft. The delivery side of the first compressor stage faces the delivery side of the second compressor stage, so that the processed gas flows through the first compressor stage generally in one direction and through the second compressor stage in the generally opposite direction. A main stream of gas processed by the compressor is sucked at the suction side of the first compressor stage and delivered at the delivery side of the second compressor stage.
In some applications, a side stream line is provided to inject a side stream gas between the delivery side of the first compressor stage and the suction side of the second compressor stage. In some applications, the side stream gas has a chemical composition different from the chemical composition of the gas sucked in the first compressor stage. For instance, the first gas processed by the first compressor stage has a molecular weight higher than the molecular weight of the side stream gas. The gas flowing through the second compressor stage, which is a mixture of the gas from the first compressor stage and the side stream gas, thus has a mean molecular weight lower than the gas flowing through the first compressor stage.
A seal arrangement is provided on the compressor shaft, between the first compressor stage and the second compressor stage, so as to reduce backflow from the last impeller at the delivery side in the second compressor stage towards the last impeller in the first compressor stage. The seal efficiency is usually such that approximately between 10-20% by weight of the gas delivered by the last impeller in the second compressor stage flows back towards the last impeller in the first compressor stage.
The first compressor stage is provided with an antisurge arrangement, usually including a recirculating bypass line including an anti-surge valve. The bypass line connects the delivery side to the suction side of the first compressor stage. When the operating point of the first compressor stage approaches the antisurge limit line, the antisurge valve is opened and a fraction of the gas flow delivered at the delivery side of the first compressor stage is recirculated towards the suction side of the first compressor stage.
When the antisurge valve opens, the gas from the side stream which leaks through the sealing arrangement between first and second compressor stages is recirculated at the suction side of the first compressor stage. As a consequence of the antisurge gas recirculation, low molecular weight gas accumulates in the first compressor stage. The mean molecular weight of the gas processed by the first compressor stage thus decreases. Since the pressure ratio of a compressor stage is dependent upon the molecular weight of the processed gas and drops when the molecular weight diminishes, antisurge recirculation causes a drop in the pressure ratio across the first compressor stage. This can eventually result in the gas pressure at the first stage suction header to increase. In some arrangements, the pressure of the gas delivered at the suction header is limited, and cannot increase at will. In this case, a drop of the pressure ratio and consequent increase of the pressure at the compressor suction side will reduce the gas flow delivered through the suction header. Under some circumstances this situation can finally lead to a loss of gas flow through the compressor train. This situation is particularly critical when two or more compressor trains are arranged in parallel and supplied by the same gas source. As a matter of fact, in this case a pressure increase at the suction side of one compressor will result in an unbalanced gas flow, with decreasing flow rate through the compressor where the pressure ratio has dropped, and increasing flow rate through the other paralleled compressor(s).
A need therefore exists for alleviating the risk of malfunctioning of a back-to-back compressor arrangement with a low molecular weight side stream.